This paper demonstrated a step by step process in designing a solar powered charging backpack that is capable of charging a mobile phone efficiently. A selection of existing products available on the market were reviewed and compared to ascertain the cost, size, and output capabilities. Next, the solar cell types and regulators were compared and their respective merits were also investigated. The charging system was then designed and tested before being integrated with the backpack. The results clearly showed that the system managed to charge the mobile phone. However, it was found that the excessive power dissipation has caused the linear regulator to generate significant heat.

1. International Journal of Power Electronics and Drive System (IJPEDS)
Vol. 9, No. 2, June 2018, pp. 848~858
ISSN: 2088-8694, DOI: 10.11591/ijpeds.v9.i2.pp848-858  848
Journal homepage: http://iaescore.com/journals/index.php/IJPEDS
Design of Solar Powered Charging Backpack
Jonas Taverne1
, Firdaus Muhammad-Sukki2
, Ahmad Syahir Ayub3
, Nazmi Sellami4
,
Siti Hawa Abu-Bakar5
, Nurul Aini Bani6
, Abdullahi Abubakar Mas’ud7
, Draco Iyi8
1,2,4,8
School of Engineering, Robert Gordon University, Scotland, United Kingdom
3
Department of Electronic & Electrical Engineering, University of Strathclyde, Glasgow, United Kingdom
5
Universiti Kuala Lumpur British Malaysian Institute, Selangor, Malaysia
6
UTM Razak School of Engineering and Advanced Technology, UniversitiTeknologi Malaysia, Kuala Lumpur, Malaysia
7
Department of Electrical and Electronic Engineering Technology, Jubail Industrial College, Saudi Arabia
Article Info ABSTRACT
Article history:
Received Sep 17, 2016
Revised Nov 22, 2015
Accepted Dec 3, 2016
This paper demonstrated a step by step process in designing a solar powered
charging backpack that is capable of charging a mobile phone efficiently. A
selection of existing products available on the market were reviewed and
compared to ascertain the cost, size, and output capabilities. Next, the solar
cell types and regulators were compared and their respective merits were also
investigated. The charging system was then designed and tested before being
integrated with the backpack. The results clearly showed that the system
managed to charge the mobile phone. However, it was found that the
excessive power dissipation has caused the linear regulator to generate
significant heat.
Keyword:
Backpack
Charging circuit
Efficiency
Regulator
Solar cell Copyright © 2018 Institute of Advanced Engineering and Science.
All rights reserved.
Corresponding Author:
Firdaus Muhammad-Sukki,
School of Engineering, Sir Iain Wood Building,
Robert Gordon University,
Garthdee Road, Aberdeen, AB10 7GJ Scotland, United Kingdom
Email: f.b.muhammad-sukki@rgu.ac.uk
1. INTRODUCTION
Solar is one of the most abundant sources of renewable energy [1–4], with the potential capability of
generating enough power to meet the demands of the entire planet. It was calculated theoretically that the
amount of sun received on earth for one day has the capability to meet the demand of the world for more than
20 years [5]. Despite the huge potential, solar photovoltaic only provides 1.2% of the world’s electricity [6].
The harnessing of solar energy is still very much ongoing with the advancements in solar technology
constantly improving. There are many applications where solar energy is utilized - from the very big
application, such as generating electricity for homes and industry from solar fields, to the very small
applications like powering the calculators. The possibilities are extensive.
One of the products that utilises the solar energy is solar powered backpack. In terms of research,
Malla et al. [7] analysed a solar powered backpack through buck boost converter to regulate the output
voltage. They carried out various simulations in MATLAB to investigate the performance of their design,
which showed good agreement when validated via experiments. Sulaiman et al. [8] investigated a solar-based
wireless charging system and found that the mobile phone charged from this setup has superior standby time
as compared to the wired charging system. Nahm [9] designed a solar backpack that comprised of 12 solar
cell which was capable of charging a Blackberry Bold model 9780 by producing a minimum voltage of 4.2V
and minimum current of 0.25A.
There are a varying number of products already on the market that utilise solar energy for storing
and charging devises. The products range in all shapes and sizes from fully integrated to attachable. As with

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the product, there is also wide range in prices from the £30 mark to well over £200. A selection of the
products is presented in Table 1. It should be noted that the prices obtained (22/02/2017) can be subject to
change. It can be seen that all products use monocrystalline solar cell which had a stated conversion rate of
22%. Generally, the power of the solar backpacks was ranging from 6W to 7W with the output voltage of
between 5V and 6V. The backpacks that come at greater cost tended to have a battery pack incorporated. The
addition of this does increase the overall weight as expected.
The purpose of the paper is to develop a different prototype of solar powered charging backpack that
is capable of charging a mobile phone efficiently. The design would have to meet certain criteria including
durability, reliability, practicality and low cost. In order for this to be achieved successfully, research, design,
testing and analysis are carried out.
Table 1. Selection of solar backpack in the market.
Sold By Watts Voltage Battery Bank Bag Size
(L*W*H)
Weight Price
(W) (V) (cm) (kg) (£)
Vitechbiz [10] 6.5 6.0 No 35*22*52 1.30 90.00
TVCMALL [11] 6.5 6.0 No 55*36*23 1.35 40.40
ECEEN [12] 7.0 -- Yes (10AH) 65*60*30 2.00 139.99
Sunsky [13] 6.5 6.0 Yes 22*8.5*32 1.80 71.99
Glorysolar [14] 7.0 5.0 No 55*36*23 1.34 57.39
The design of the charging circuit and the solar cell will need to meet the requirements of the mobile
phone. The phone that will be used as a test is Vodafone’s Smart First 7. The battery is Li-ion with a capacity
of 1400mAh, a nominal voltage of 3.7V and a charge up voltage of 4.2V. The phone operates a micro USB
charger connection on the phone side and a USB version 2.0 connection to charger [15]. The charging circuit
should have the appropriate compatible USB connection.
2. CHARGING CIRCUIT
Within a charging circuit it is necessary to have a regulated voltage output to supply the charging
capabilities to the load. Therefore, it is important to look at the different types of voltage regulators available.
The two main types are switching and linear. Switching regulators can be broken down into buck types (step-
down), boost (step-up) and inverter types.
The main stipulation for the charging circuit is to regulate an output voltage of 5V to satisfy the
charging requirement for the phone battery. When comparing the linear with the switching regulator it is
important to establish the most practical circuit with relation to efficiency and cost. The switching circuits
have higher complexity when it comes to the external components. In comparison, the linear circuits’
external components are reduced due to the linear operation of the regulator. The reduction in complexity
relates to a cheaper circuit cost. However, the switching types have higher efficiencies which are at the
expense of circuit complexity. The switching types are also capable of boosting as well as operating in buck
mode. Despite this, switching regulators have a higher output voltage ripple owing to their switching nature,
produce electromagnetic interference (EMI) and noise and are generally more expensive than linear
regulators [16].
The main advantage of the buck and boost converters is the efficiency rates which can be greater
than 90% [16]. This is due in part to the inductor which provides current to the load during off periods of the
MOSFET instead of dissipating its stored energy as heat. This is also the case for the capacitor which charges
during on periods and then supplies the load during the off periods and once the inductor has discharged.
Some of the main advantages of the linear type are a lower output voltage ripple, transient response to any
changes with the load, and low EMI and noise. The low noise and low EMI is attributed to the non-switching
characteristics of the circuit and no inductors in operation. Linear regulators (LRs) have some disadvantages
relating to their low efficiency, heat sink is required, unable to step down voltage.
Taking the advantage and disadvantages into consideration, the LR will be used as the voltage
regulator. This is due to the lower cost associated with the external circuitry and regulator itself, EMI and
noise reduction, and low output voltage ripple [17]. Consideration into operating with a low dropout
regulator (LDO) was taken and despite being more efficient than LR, it was not deemed feasible. This was
based on the fact that the LDO operates with low voltage drops (typically in mV) [16]. This would become
problematic when the weather conditions are such that the solar panel supply increases/decreases by one or
more volts.

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The circuit comprises of a voltage divider to regulate the output voltage. A diode was used to protect
the solar cell from reverse charge of the battery. The circuit, once designed will undergo tests and
improvements made if necessary.
The first and most important component is the LR. Following the specifications of the mobile phone
battery it is necessary to have an output of 5V from the circuit as previously discussed. The input voltage will
need to be of sufficient magnitude to allow for the regulated output voltage with relation to the minimum
dropout voltage of the chosen regulator.
Several LR were looked at with relation to the adjustable output voltage range and the maximum
output current. TS317CM was found to be suitable. The output voltage was adjustable over a range of 1.25V
to 37V and a maximum output current of 1.5A for package type TO-220. The regulator has several key
internal protection feature such as thermal overload, short circuit current limiting constant with temperature
and output transistor safe area compensation [16]. With these internal protection features it meant that
external protection circuitry was reduced.
As stated, the regulated output voltage is set by a voltage divider connected to the adjust pin of
TS317CM. can be a fixed resistor or a variable resistor to allow for variations of . LTspice was used
as the simulation software package to verify the output voltage. This was achieved by setting the resistance
values in accordance to the calculated values and running the simulation. The simulated circuit and
corresponding results can be found in Figures 1 and 2.
Figure 1. Simulated circuit design
Figure 2. Simulated output voltage.
For simulation, a resistor was used as a load. Diode protects against reverse charge from the
battery. The diodes rating needs to have a breakdown voltage that will maintain the correct biasing
throughout circuit operation. The diode chosen for the constructed circuit was 1N4004 as this was more than
capable so satisfying the operational criteria [17].
The value of was found from both calculation and simulation results. The equation used to find
as stated in the datasheet was the following [16]:

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(1)
where , with typical value [16], and &
respectfully. The resistor ratio was found from simplifying Equation 1 by neglecting the addition of
giving:
(2)
Equation 2 can then be rearranged to find as shown in Equation 3.
(3)
For an output voltage of 5V gives a ratio of 3, as shown:
(4)
The output voltage was then calculated using Equation 2 with all the parameters stated. The
calculated result of which was just over the required operating voltage of was acceptable.
Through simulation was shown to be as seen in Figure 2. It is clear to see form the specification
that the results backup the calculated result.
An input capacitor will need to be used if the solar panel is connected an appreciable distance from
circuit, as specified by the datasheet. The advantage of connecting a capacitor across the input ( ) will
reduce the sensitivity to input line impedance [16]. Capacitor improves the transient response of the
circuit which is specified in the datasheet along with the value of capacitance [16]. The voltage tolerance of
the capacitors is such that the voltage values are greater than the circuit voltages as to maintain full
capacitance operation value.
3. SELECTION OF SOLAR CELL
Selecting the appropriate solar cell/panel depends mainly on efficiency and cost. Tables 2 provides
some of the advantages and disadvantage of the three different solar cell types. It can be seen from Table 2
that, in terms of efficiency, monocrystalline is the most efficient with polycrystalline and amorphous cell
following second and third respectfully. One of the main reasons for the amorphous type having a
significantly lower efficiency is because, when compared to monocrystalline, it is still a fairly new
technology [18]. However, it is still necessary as comparison to the more established types.
Table 2. Advantages and disadvantages of solar cells
Type of cell Advantages Disadvantages
Monocrystalline High efficiency rates 15% to 20% Expensive
Space efficient Silicon waste during manufacture
Long operational lifespan More efficient in warmer climates, decreased performance as
temperature rises
Generally perform better than similar rated
polycrystalline
Polycrystalline Not as expensive as monocrystalline Not as space efficient as monocrystalline
Less waste silicon Efficiency rates of 13% to 16% due to lower silicon purity
Amorphous Less silicon needed Low efficiency rates of 10% to 12%
Substrates can be made from inexpensive material Thin film tends to degrade faster
Can be sprayed on flexible and light material Larger coverage needed than crystalline based cells
Performs relatively well under poor light/not
affected by shading issues
Larger coverage needed than crystalline based cells
Monocrystalline has a lot of strong attributes, not only in high efficiency rates but also with its
efficiency relating to space. With monocrystalline, the high rates of efficiency mean that the overall size of
the cell/panel can be reduced. The general life-span of monocrystalline is greater than the other three types,

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given that manufacturers and sellers alike attaching a 25-year warranty [18]. The higher efficiency and longer
life-span is represented in the cost, with monocrystalline being the more expensive.
Polycrystalline and amorphous types lack the higher efficiency rates of the monocrystalline. The
space efficiency in the polycrystalline is less than that of the monocrystalline but does consume less silicon
during manufacturing. The amorphous type, again, requires less material which is cheaper. The construction
of amorphous solar cells makes them more flexible and lightweight. The operation of amorphous is such that
it behaves ‘good’ under poor light and operates effectively in colder conditions [18].
The backpack that is to be used has an allowable amount of space which needs to come into
consideration when the choice is made. This will also have an impact as the limited space will mean an
efficient solar type will need to be selected. It is therefore determined that monocrystalline will be used.
Despite the higher costs incurred, the reasons stated above outweigh the higher costs.
The rating of the solar cell/panel is determined by the type of voltage regulator (switching or linear)
used in the charging circuit. As the selected regulator is of the linear type, there needs to be compensation for
the minimum allowable voltage drop as stated by the regulator. Therefore, the rating of the solar panel needs
to take this factor into consideration. The LR has an allowable minimum volt drop of 2V which when added
with the USB voltage of 5V relates to a minimum panel rating of 7V. After some consideration, two 6V 6W
rated monocrystalline solar cells connected in series are used to supply more than 7V to the charging circuit.
4. TESTING
Initial testing of the circuit was carried out using the designed circuit to charge the mobile phone.
The initial battery percentage was recorded and checked every 5 minutes over an appreciable period. The
input and output voltages as well as the current to the load were recorded. The test was carried out under both
cloudy and clear days to achieve a clear comparison of the operation of the circuit. During both test
conditions the cells were placed as close to 90⁰ to the sun as possible with no obstructions.
Figure 3. Testing setup
Figure 3 shows the test setup. The testing was carried out in a number of days. The following is the
test carried out was under cloudy weather conditions on date 06/04/017 in Aberdeen, Scotland. The panel is
assume to be static, and was place in an open space with no shading from nearby buildings. The start charge
of the battery was 2%. Figure 4 shows the increasing charge of the battery over the test time. As previous,
there is little increase within the first twenty minutes of charge. Within the next forty minutes of charge there
is a significant increase in the batteries capacity. From time, 16:05 onwards, the increase becomes more
gradual as the battery capacity increases.

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Figure 4. Battery capacity test
As can be seen from Figure 5, the current did not deviate from its max output of throughout
the test. This was due to the input voltage maintaining above the allowable volt drop of the circuit, as
observed from Figure 6.
Figure 5. Load current test
The efficiency can be calculated from the following Equation (5) [16]:
(5)
Figure 6. Input and output voltage comparison test

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Figure 7. Efficiency test
From Figure 7, it clearly shows the efficiency of the circuit to be around the mark. The
lowest efficiency was recorded a and the highest at . The result of which meant a large
amount of power was dissipated as heat causing TS317CM to heat up significantly. The tests have shown that
the circuit was successful in charging the phone. It is also clear to see the importance of matching the correct
rating of cell to the characteristics of the circuit. The two cells test showed that the higher input voltages
would result in lower efficiencies and cause excessive heating of the LR. In the future, it is suggested that a
single cell is used to provide both the higher efficiencies and appropriate input voltage levels to maintain
correct operation of the charging circuit.
5. INTEGRATION OF COMPONENTS
The most important consideration during integration is to maintain the integrity of the solar panel
and charging circuit. The system needs to be robust but still operate effectively. The two main options for
integration is either fully integrated or partially.
Requirements for full integration:
a. Water-resistant
b. Shock resistance
c. Aesthetically acceptable
Requirements for partial integration:
a. Easily attachable
b. Small package footprint for storage
c. Robust
d. Aesthetically acceptable
e. Water-resistant
There are several advantages and disadvantages to both systems. The benefit with a fully integrated
system is that it comes as a complete package with no need to attach the panel. In contrast the partial method
allows for the product to be removed completely if not in use. The product can then be stored safely without
the possibility of getting damaged. Each cell used has the following dimensions: 165mm x 165mm. As stated
above the system would have to adhere certain requirements.
The design that was looked at was using the 2 cells as shown in Figure 8. The design was meant to
be such that the panels could be attached and removed when necessary. For this arrangement, a book like
design was constructed. There were no costs during the design/construction of the integration process as all
materials were recycled/re-purposed materials.

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Figure 8. Two-cell series connection integration design
The design could then be attached using Velcro straps. The panels are protected by a cardboard
backing and edging to provide some shock absorption. The cardboard frame is enclosed in a denim material
which will allow a degree of ware-and-tear. The whole panel design, when removed, can be folded and stored
away protecting the panels. This design does have some disadvantages when it comes to overall size which
does impact the aesthetics. However, there is the added advantage that this design can be removed and placed
in the optimum position when stationary. An example of a final product of a solar powered charging
backpack integration is presented in Figure 9.
It is also good to estimates the cost to produced this prototype. Table 3 shows the breakdown cost of
the components to produce the backpack. Other costs such as delivery cost, labour cost etc. are excluded
from the calculation. The cost to produce the prototype was approximately £38.38. If we consider to mass
produce this product, its production cost can be further reduced and could potentially compete with the
existing products available in the market.
Figure 9. Final product – using one cell integration
Table 3. Breakdown cost to produce the prototype (in £).
Components Cost Quantity Total
Solar Panel 12.49 1 12.49
TS317CM 0.59 1 0.59
Heat Sink 1.29 1 1.29
USB type A 3.42 1 3.42
Backpack 10.00 1 10.00
Circuit Box 5.49 1 5.49
Sundries* 5.00 - 5.00
TOTAL 38.28
*This includes solder, wire and small cost components like resistors, diode and capacitors.
6. CONCLUSION

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There have been advances in solar technology with relation to the different solar cell; however
monocrystalline still holds the higher efficiency rates which made it the logical choice when selecting the
appropriate type. This was reflected in the research and tests carried out. It must be concluded that despite the
higher efficiency ratings of monocrystalline, amorphous solar cells have the ideal physical properties with
relation to flexibility. With greater efficiencies in amorphous cells would result in a more adaptable product.
The overall design and operation of the circuit managed to successfully charge the mobile phone
with acceptable efficiency ratings. The circuitry was kept simple but effective as a result of utilising the LR.
This meant that the cost was kept to a minimum.
The tests carried out under varying weather conditions clearly showed that the system managed to
charge the mobile phone. However, it was found that the excessive power dissipation has caused the LR to
generate significant heat. In the future, it is suggested that a single cell is used to provide both the higher
efficiencies and appropriate input voltage levels to maintain correct operation of the charging circuit.
The integration of the system with the backpack was vital as the aesthetics of the final product was
important to uphold. The choice of making the product a partial integration was due to versatility. The system
could be removed and integrated with other backpacks easily making it a more attractable solution.
ACKNOWLEDGEMENTS
The author would like to thank Ministry of Higher Education (MOHE), Malaysia and Universiti
Teknologi Malaysia (UTM) (Research cost centre no. Q.K130000.2540.16H95) for funding part of this
project.
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BIOGRAPHIES OF AUTHORS

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Jonas Taverne obtained his HND fin Electrical and Electronic Engineering from North East
Scotland College in 2015. Subsequently, he pursued his BEng in the same are in Robert Gordon
University and graduated in 2017. His research interest includes solar energy and renewable
technology.
Firdaus Muhammad-Sukki received his MEng degree in Electrical and Electronic Engineering
from Imperial College, London in 2006. In 2009, he pursued his postgraduate study in Glasgow
Caledonian University from which he obtained his PhD in 2013. Dr. Muhammad Sukki is a
Chartered Engineer, a member of the Institution of Engineering and Technology (IET) and an
associate of the City and Guilds, London (ACGI). Dr. Muhammad Sukki is currently a Lecturer
in Robert Gordon University, United Kingdom. His research interest lies in designing optical
concentrator, which can be used in many applications including creating a low cost solar
photovoltaic system, illumination, heating and cooling of buildings, energy harvesting,
desalination, medical devices, waste water treatment etc. On top of that, Dr. Muhammad Sukki
also carried out a number of non-technical research including market trend and financial analysis
related to renewable technologies for countries such as Malaysia, Japan, Nigeria, Cameroon and
United Kingdom. He has excellent track record in collaborating with research universities
around the world. He has published numerous articles in high impact factor journals (e.g. Nature,
Renewable & Sustainable Energy Reviews, Applied Energy, Energy etc.), as well as presenting
in various conferences related to his area. Prior to joining the academia, he was a communication
engineer in Malaysia’s largest telecommunication company.
Ahmad Syahrir Ayub received B. Eng (Hons) in Electrical & Electronic Engineering from
Cardiff University, Wales, UK in 2007. In 2008, he was working in the area of cable diagnostics
in the High Voltage Technologies (HVT) Group at University of Strathclyde, Glasgow,
Scotland. Currently, he is pursuing his PhD in Lightning Protection of Wind Turbine Blades at
University of Strathclyde, Glasgow, Scotland. His areas of interest include lightning protection
and cable diagnostics. He is a Student Member of both IET and IEEE.
Nazmi Sellami received an MEng degree in Mechanics and Energetics from Ecole Polytechnique
Universitaire de Marseille, France (2004). After a period of work in the industry at Oxford
Instruments, he returned to University to obtain an MSc in Renewable Energy Engineering from
Heriot-Watt University, Edinburgh with distinction (2009) and was offered a scholarship to
commence a PhD at the same establishment. He completed his thesis on the design of novel solar
concentrator for building integration in 2013. Before joining RGU as a lecturer in Mechanical
Engineering, Dr Sellami worked as a research fellow at Exeter University and an Associate
Professor in Mechanical Engineering at Heriot-Watt University, Dubai Campus. Dr Sellami has
been awarded an “Outstanding Impact award in Sustainable Future” with a group of researchers
at the University of Exeter. His research interests include the characterisation and design of
thermal and photovoltaic solar devices for various types of applications, and especially for
building integration.
Siti Hawa Abu-Bakar obtained her BEng in Information Systems Engineering from Imperial
College, London in 2006 under the MARA scholarship. In 2007, she joined Prokhas Sdn. Bhd., a
wholly-owned company under the Ministry of Finance Malaysia as the Executive Trainee. From
2008 onwards, she becomes a unit trust consultant for Public Mutual Bhd. In 2011, she was
awarded studentships from GCU and Scotland's Energy Technology Partnership (ETP) to pursue
her post graduate study in solar technology from which she obtained her PhD in 2016. Dr Abu-
Bakar is an associate of the City and Guilds, London (ACGI). Her interests include solar PV,
solar thermal, financial planning and investment analysis.

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Nurul Aini Bani received her M.Eng degree in Electrical Engineering from University of
Southampton, UK in 2006 where she received two academic awards for outstanding academic
performance. She received her PhD degree in Electrical Engineering from Universiti Teknologi
Malaysia, Malaysia in 2016. Her research interests include polymeric insulation material, space
charge measurement, high voltage cable, renewable energy and rehabilitation engineering. She
has authored and co-authored several papers in various technical journals and conference
proceedings. She is now a lecturer in the Department of Engineering, UTM Razak School of
Engineering and Advanced Technology (RSEAT).
Abdullahi Abubakar Mas’ud received his BEng degree in Electrical Engineering in 1999 and
Msc in Electronics and Communication in 2006, both from Ahmadu Bello University, Zaria,
Nigeria. He also obtained a PhD in Electrical Engineering in 2013 from the Glasgow Caledonian
University, Scotland, UK, under the supervision of Prof. Brian Stewart and Prof. Scott
McMeekin. In 2002 he became an assistant lecturer in the Faculty of Engineering at the Ahmadu
Bello University, Zaria, Nigeria. In 2013 he joined Jubail Industrial College, Saudi Arabia, and
is currently an Assistant Professor in the Department of Electrical and Electronic Engineering.
Dr. Abdullahi has published a number of papers in high impact factor journals and conference
proceedings in the area of high voltage partial discharge and renewable energy. Dr. Abdullahi A
Mas’ud is a Member of the IET, International association of Engineers and a registered Engineer
(COREN) Nigeria.
Draco Iyi obtained his MSc and PhD degrees in Mechanical Engineering at Northumbria
University in 2009 and 2013 respectively. His doctoral research focused on buoyancy driven
turbulent flows associated with radiation in a high density enclosure. Dr Draco joined Robert
Gordon University in August 2014 as a full time member of the School of Engineering. Prior to
this, he was a lecturer in the department of Mechanical and Construction Engineering at
Northumbria University, Newcastle upon Tyne, where he was involved in research, teaching and
learning activities at undergraduate and postgraduate level. Dr Draco has been involved in
various consultancy activities with clients on developmental projects in thermos-fluid, CFD and
process design engineering. Dr Draco also had several industrial work experiences and currently
working on the design of experimental techniques to supply reliable data for the validation of
CFD codes and techniques.